The invention relates to a method for determining the spatial distribution of the magnitude of the radio frequency (=RF) transmission field B1 in a magnetic resonance imaging (=MRI) apparatus, wherein the method comprises performing an MRI experiment in which a B1-sensitive complex image, with a real part and an imaginary part, of a sample is obtained, wherein the phase distribution, with the phase determined by the arc tangent function, within the B1-sensitive complex image depends on the spatial distribution of the magnitude of the field B1.
Such a method is known from U.S. Pat. No. 6,268,728 B1, see Ref. [3].
In a MRI (=Magnetic Resonance Imaging) apparatus, the signal used for image reconstruction is generated by the component of the nuclear magnetization that is transverse relative to the main magnetic field axis. The transverse magnetization is created by exciting the nuclear spins with one or several pulses of a transverse magnetic field oscillating with the resonance frequency, typically in the radio wave range. This so called radio-frequency (RF) field, commonly denoted B1, is produced by an electrical circuit called transmission coil that is designed to obtain a possibly uniform distribution of the B1 field strength in the volume of interest. However, due to design imperfections and due to the electromagnetic properties of the object being imaged, the effective RF field is not uniform, leading to a spatially non-uniform excitation. This, in turn, leads to non-uniform image intensity and contrast.
When the spatial distribution of the effective RF field (i.e. the B1 map) is known, it is possible to correct the image intensity digitally to compensate for the non-uniform excitation. It is also possible to design two- or three-dimensional RF pulses that can take the B1 inhomogeneity into account and produce a uniform excitation. The knowledge of B1 maps is also essential in the technique of parallel excitation with several circuits to obtain a homogeneous excitation in the volume of interest. For this reason, there has been a considerable interest in the development of B1 mapping methods.
Several methods for B1-field mapping have been proposed using signal intensity as the indicator of the B1 strength. These involve comparisons of image intensities obtained with different RF intensities [1] and comparisons of images acquired with a single RF field intensity but various repetition times [2]. The drawback of these methods is that the signal intensity vs. B1 relation is rather complex and depends on relaxation properties of the object, which are spatially non-uniform and may falsify the result.
An interesting alternative to the intensity-based B1-mapping methods is the phase-based method [3, 4]. The transverse magnetization is generated by a pair of RF pulses in such a way that its direction in the rotating frame (coordinate system rotating around the main magnetic field with the resonance frequency), and thus the signal phase, depends on the strength of the B1 field. More specifically, in the method of [3, 4], a non-selective pulse with a flip angle of 2α about the x axis is followed by a non-selective pulse with a flip angle of α about the y axis. The amplitude of the signal is irrelevant in this method as long as it is high enough to allow the calculation of the phase, and the dependence of the measurement on the relaxation properties of the object is avoided.
Both the intensity-based and the phase-based B1 mapping methods can measure the B1 field in a limited range of values. The lower limit of this range is determined by the noise: an RF field intensity below this limit will generate transverse magnetization that is too low to be detected in the presence of noise. The upper limit is given by the field strength which flips the magnetization by 180 degrees and does not produce detectable signal either.
Adiabatic RF pulses are a well known tool to perform rotations of the magnetization independently on the amplitude of the B1 field [5]. These pulses involve a sweep of the RF frequency during the application of the pulse. The sweep can either start on-resonance and move to off-resonance or the other way round, as in the Adiabatic Half-Passage (AHP) pulses, or it can by symmetric, starting on one side of the resonance frequency and moving to the other side, as in the Adiabatic Full-Passage (AFP) pulses. Possible applications of adiabatic pulses include the creation of a free induction decay signal using a single AHP pulse, or the generation of a spin echo signal by a standard excitation followed by an AFP refocusing pulse. In both cases, the amplitude of the transverse magnetization is maximal for any B1 amplitude above a certain threshold. The phase of the magnetization, however, strongly depends on the B1 amplitude. This dependence has been regarded as an unwanted effect since it may cause a cancellation of signals emitted from different object positions. Various composite AHP/AFP pulse sequences have therefore been proposed to achieve B1-insensitive rotations of the magnetization [6].
K. Shultz et al., Proc. Intl. Soc. Mag. Reson. Med. 16 (2008), p. 1245, describes adiabatic B1 mapping for RF current density imaging, wherein a measured signal amplitude is analysed.
C. A. Meriles et al., Journal of Magnetic Resonance 164 (2003), 177-181, describes the use of a gradient of the B1 field to produce a desired phase modulation pattern of the transverse magnetisation.
U.S. Pat. No. 6,750,649 B1 describes the design of adiabatic pulses. An adiabatic pulse starting off-resonance and ending on-resonance is used to map an RF amplitude by determining a phase of a return signal as a function of spatial location.
It is the object of the invention to provide a simple method for mapping the B1 field of a magnetic resonance imaging apparatus with an improved accuracy and a wider measurement range.